Polycrystalline gallium-nitride self-supporting substrate and light-emitting element using same

ABSTRACT

Provided is a self-supporting polycrystalline GaN substrate composed of GaN-based single crystal grains having a specific crystal orientation in a direction approximately normal to the substrate. The crystal orientations of individual GaN-based single crystal grains as determined from inverse pole figure mapping by EBSD analysis on the substrate surface are distributed with tilt angles from the specific crystal orientation, the average tilt angle being 1 to 10°. There is also provided a light emitting device including the self-supporting substrate and a light emitting functional layer, which has at least one layer composed of semiconductor single crystal grains, the at least one layer having a single crystal structure in the direction approximately normal to the substrate. The present invention makes it possible to provide a self-supporting polycrystalline GaN substrate having a reduced defect density at the substrate surface, and to provide a light emitting device having a high luminous efficiency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of PCT/JP2015/58752 filedMar. 23, 2015, which claims priority to Japanese Patent Application No.2014-71342 filed Mar. 31, 2014, PCT/JP2014/64388 filed May 30, 2014,U.S. patent application Ser. No. 14/499,688 filed Sep. 29, 2014, andJapanese Patent Application No. 2014-241013 filed Nov. 28, 2014, theentire contents all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a self-supporting polycrystallinegallium nitride substrate and a light emitting device including such aself-supporting polycrystalline gallium nitride substrate.

2. Description of the Related Art

As light emitting devices such as light emitting diodes (LEDs) in whicha single crystal substrate is used, light emitting devices in whichvarious gallium nitride (GaN) layers are formed on sapphire (α-aluminasingle crystal) are known. For example, those having a structure formedby stacking on a sapphire substrate an n-type GaN layer, a multiplequantum well (MQW) layer in which a quantum well layer composed of anInGaN layer and a barrier layer composed of a GaN layer are alternatelystacked, and a p-type GaN layer in this order are in mass production.Moreover, a multi-layer substrate suitable for such use is alsoproposed. For example, Patent Document 1 (JP2012-184144A) proposes agallium nitride crystal multi-layer substrate including a sapphire basesubstrate and a gallium nitride crystal layer formed by crystal growthon the substrate.

When a GaN layer is formed on a sapphire substrate, dislocation islikely to occur because the lattice constant and the coefficient ofthermal expansion of the GaN layer do not match with those of sapphire,which is a foreign substrate. Moreover, since sapphire is an insulatingmaterial, it is not possible to form an electrode on its surface, and,therefore, it is not possible to configure a light emitting devicehaving a vertical structure that includes electrodes on the front andback of the device. Accordingly, LEDs in which various gallium nitride(GaN) layers are formed on a GaN single crystal have been attractingattention. Since a GaN single crystal substrate is made of the same typeof material as a GaN layer, the lattice constants and the coefficientsof thermal expansion are likely to match, and higher performance can beexpected than the case where a sapphire substrate is used. For example,Patent Document 2 (JP2010-132556A) discloses a self-supporting n-typegallium nitride single crystal substrate having a thickness of 200 μm orgreater.

CITATION LIST Patent Documents

Patent Document 1: JP2012-184144A

Patent Document 2: JP2010-132556A

SUMMARY OF THE INVENTION

However, single crystal substrates in general have small areas and areexpensive. In particular, while there are demands for reduction ofproduction costs of LEDs in which large-area substrates are used, it isnot easy to mass-produce large-area single crystal substrates, and doingso results in even higher production costs. Accordingly, an inexpensivematerial that can be an alternative material for single crystalsubstrates of gallium nitride or the like is desired. Previously, thepresent inventors successfully produced a self-supportingpolycrystalline gallium nitride substrate that meets such demands (thisis neither publicly known nor constitutes prior art), but furtherimprovements are desired in the crystallinity of the self-supportingpolycrystalline gallium nitride substrate.

The inventors have currently found that by arranging the constitutivegrains of the self-supporting polycrystalline gallium nitride substrateto have a specific crystal orientation in a direction approximatelynormal to the substrate and, at the same time, to becrystallographically tilted so as to have an average tilt angle within apredetermined range, it is possible to reduce the defect density at thesubstrate surface. Moreover, the present inventors also found that lightemitting devices formed from such a self-supporting polycrystallinegallium nitride substrate have a higher luminous efficiency than lightemitting devices formed from a self-supporting polycrystalline galliumnitride substrate in which the constitutive grains do not have a tiltedcrystal orientation.

Therefore, an object of the present invention is to provide aself-supporting polycrystalline gallium nitride substrate having areduced defect density at the substrate surface. Another object of thepresent invention is to provide a light emitting device having a highluminous efficiency, by using such a self-supporting polycrystallinegallium nitride substrate.

According to an aspect of the present invention, there is provided aself-supporting polycrystalline gallium nitride substrate composed of aplurality of gallium nitride-based single crystal grains having aspecific crystal orientation in a direction approximately normal to thesubstrate, wherein

-   -   crystal orientations of individual gallium nitride-based single        crystal grains as determined from inverse pole figure mapping by        electron backscatter diffraction (EBSD) analysis performed on a        substrate surface are distributed with various tilt angles from        the specific crystal orientation, wherein an average tilt angle        thereof is 1 to 10°.

According to another aspect of the present invention, there is provideda light emitting device comprising:

-   -   the self-supporting polycrystalline gallium nitride substrate        according to the foregoing aspect of the present invention; and    -   a light emitting functional layer formed on the substrate,        wherein the light emitting functional layer has at least one        layer composed of a plurality of semiconductor single crystal        grains, wherein the at least one layer has a single crystal        structure in a direction approximately normal to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional diagram showing one example of avertical light emitting device produced using the self-supportingpolycrystalline gallium nitride substrate of the present invention.

FIG. 2 is an inverse pole figure map of the plate surface of aself-supporting polycrystalline gallium nitride substrate obtained inExample A1.

FIG. 3 is a graph showing the frequency of the tilt angles from thec-axis direction of grains constituting the outermost surface, which istabulated from inverse pole figure mapping in Example A1.

DETAILED DESCRIPTION OF THE INVENTION Self-Supporting PolycrystallineGallium Nitride Substrate

The gallium nitride substrate of the present invention can take the formof a self-supporting substrate. In the present invention, the“self-supporting substrate” means a substrate that does not becomedeformed or damaged by its own weight when handled and that can behandled as solid matter. The self-supporting polycrystalline galliumnitride substrate of the present invention is usable as a substrate forvarious semiconductor devices such as light emitting devices, and, inaddition, it is usable as a component or a layer other than a substrate,such as an electrode (which may be a p-type electrode or an n-typeelectrode), a p-type layer, or an n-type layer. In the followingdescription, advantages of the present invention may be described by wayof a light emitting device as an example, which is one of the principalapplications, but similar or analogous advantages are also applicable toother semiconductor devices as long as such advantages are nottechnically contradictory.

The self-supporting polycrystalline gallium nitride substrate of thepresent invention is composed of a plurality of gallium nitride-basedsingle crystal grains having a specific crystal orientation in thedirection approximately normal to the substrate. In this self-supportingpolycrystalline gallium nitride substrate, the crystal orientations ofindividual gallium nitride-based single crystal grains as determinedfrom inverse pole figure mapping by electron backscatter diffraction(EBSD) analysis performed on the substrate surface (plate surface) aredistributed with various tilt angles from a specific crystal orientation(e.g., the orientation of the c-axis, a-axis, etc.), and the averagetilt angle thereof is 1 to 10°. EBSD is a known technique that providesinformation on the crystal system and crystal orientation of acrystalline sample by irradiating the sample with an electron beam toreveal Kikuchi diffraction pattern, i.e., an EBSD pattern, resultingfrom electron backscatter diffraction at the sample surface. The EBSD incombination with a scanning electron microscope (SEM) providesinformation on the crystal system and crystal orientation distributionof a microscopic region through determining and analyzing the EBSDpattern while electron-beam scanning. As described above, arranging theconstitutive grains of the self-supporting polycrystalline galliumnitride substrate to have a specific crystal orientation in thedirection approximately normal to the substrate and, at the same time,to be crystallographically tilted so as to have an average tilt anglewithin a predetermined range makes it possible to reduce the defectdensity at the substrate surface. Although the reason of such a reduceddefect density is not clear, it is presumed that a slight tilt of thecrystal orientation of the gallium nitride-based single crystal grainsis likely to promotes defects, which are caused by lattice mismatch withthe base substrate (typically an oriented polycrystalline sintered body)used during production, to merge with each other and disappear withingrains. It is also considered that defects develop in a tilted mannerrelative to the direction normal to the substrate due to the slight tiltof the crystal orientation and disappear at grain boundaries.

In addition, forming a light emitting device from such a self-supportingpolycrystalline gallium nitride substrate in which the constitutivegrains have a tilted crystal orientation makes it possible to attain ahigher luminous efficiency than a light emitting device formed from aself-supporting polycrystalline gallium nitride substrate in which theconstitutive grains do not have a tilted crystal orientation. Althoughthe reason of such a higher luminous efficiency is not clear, it ispresumed that, as described above, the defect density of the substrateis small and, therefore, the defect density of a light emittingfunctional layer grown thereon is also small, thus resulting in a higherluminous efficiency. Moreover, the light emitting functional layerformed on the substrate also acquires a structure that have a tiltedcrystal orientation and, presumably, the efficiency of light extractionis increased accordingly.

The gallium nitride-based single crystal grains constituting theself-supporting polycrystalline gallium nitride substrate have aspecific crystal orientation in the direction approximately normal tothe substrate. This specific crystal orientation may be any crystalorientation (e.g., the c-plane, a-plane, etc.) that gallium nitride mayhave. For example, when the gallium nitride-based single crystal grainshave a c-plane orientation in the direction approximately normal to thesubstrate, each constitutive grain at the substrate surface is disposedsuch that its c-axis extends in the direction approximately normal tothe substrate (that is, the c-plane is exposed to the substratesurface). While the gallium nitride-based single crystal grainsconstituting the self-supporting polycrystalline gallium nitridesubstrate have a specific crystal orientation in the directionapproximately normal to the substrate, individual constitutive grainsare slightly tilted at various angles. That is, although the substratesurface as a whole exhibits a specific crystal orientation in thedirection approximately normal to the substrate, the crystalorientations of individual gallium nitride-based single crystal grainsare distributed with various tilt angles from the specific crystalorientation. This unique oriented state can be assessed from inversepole figure mapping by EBSD (see, for example, FIG. 2) performed on thesubstrate surface (plate surface) as described above. That is, thecrystal orientations of individual gallium nitride-based single crystalgrains as determined from inverse pole figure mapping by EBSD performedon the substrate surface are distributed with various tilt angles fromthe specific crystal orientation, and the average value of the tiltangles (the average tilt angle) thereof is 1 to 10°, preferably 1 to 8°,and more preferably 1 to 5°. It is preferable that no less than 80% ofthe gallium nitride-based single crystal grains subjected to inversepole figure mapping by EBSD have a tilt angle within a range of 1 to10°, and more preferably no less than 90%, even more preferably no lessthan 95%, and particularly preferably no less than 99% of the grainshave a tilt angle within the aforementioned range. Such a tilt angledistribution as above results in a significantly reduced defect density.Moreover, it is preferable that the tilt angles of the galliumnitride-based single crystal grains are distributed according toGaussian distribution (which is also referred to as the normaldistribution), and the defect density is significantly reducedaccordingly.

It is preferable that the self-supporting polycrystalline galliumnitride substrate has a reduced defect density due to the tilting of theconstitutive grains as described above. For example, the self-supportingpolycrystalline gallium nitride substrate preferably has a defectdensity of 1×10⁴ defects/cm² or less, more preferably 1×10³ defects/cm²or less, even more preferably 1×10² defects/cm² or less, particularlypreferably 1×10¹ defects/cm² or less, and, most preferably,substantially no defects (i.e., approximately 0 defects/cm²). The defectdensity can be determined by counting the number of dark spots, whichappear darker than the surroundings due to their weak light emission ina cathode luminescence (CL) method, as dislocations appearing on thesubstrate surface. The CL method is a known technique for detectinglight emitted when irradiating a sample with an electron beam and makesit possible to analyze the state of a spot while verifying the positionof the spot on an SEM image. Measurement by the CL method can beperformed using, for example, an SEM (scanning electron microscope)equipped with a cathode luminescence detector.

It is preferable that the self-supporting polycrystalline galliumnitride substrate has a single crystal structure in the directionapproximately normal to the substrate. In this case, it can be said thatthe self-supporting polycrystalline gallium nitride substrate iscomposed of a plate composed of a plurality of gallium nitride-basedsingle crystal grains having a single crystal structure in the directionapproximately normal to the substrate. That is, the self-supportingpolycrystalline gallium nitride substrate is composed of a plurality ofsemiconductor single crystal grains connected two-dimensionally in ahorizontal plane direction, and, therefore, can have a single crystalstructure in the direction approximately normal to the substrate.Accordingly, although the self-supporting polycrystalline galliumnitride substrate is not a single crystal as a whole, theself-supporting polycrystalline gallium nitride substrate has a singlecrystal structure in terms of local domains. Such a configurationenables satisfactory characteristics to be attained when producingdevices having light emitting functions and devices such as solar cells.Although the reason of this is not clear, this is considered to be theeffect resulting from the transparency/translucency and light extractionefficiency of the polycrystalline gallium nitride substrate. Moreover,the use of a gallium nitride substrate provided with electroconductivityby introducing a p-type or n-type dopant makes it possible to achieve alight emitting device having a vertical structure and, thereby, anincreased luminance. In addition, a large-area surface light emittingdevice for use in surface emitting lightings or the like can be achievedat low cost. In particular, when a vertical LED structure is producedusing the self-supporting polycrystalline gallium nitride substrate ofthis embodiment, because the plurality of gallium nitride-based singlecrystal grains constituting the self-supporting substrate have a singlecrystal structure in the direction approximately normal to thesubstrate, highly resistive grain boundaries do not exist in electricalcurrent paths, and as a result, preferable luminous efficiency isexpected. In this regard, in the case of an oriented polycrystallinesubstrate in which grain boundaries exist also in the direction normalto the substrate, highly resistive grain boundaries exist in electricalcurrent paths even when a vertical structure is formed, and thus thereis a possibility of impaired luminous efficiency. From these viewpoints,the self-supporting polycrystalline gallium nitride substrate of thisembodiment can be preferably used also for a vertical LED structure.Moreover, since grain boundaries do not exist in electrical currentpaths, the self-supporting polycrystalline gallium nitride substrate isapplicable not only to such light emitting devices but also to powerdevices, solar cells, etc.

Preferably, the plurality of gallium nitride-based single crystal grainsconstituting the self-supporting substrate have crystal orientation thatis mostly aligned in the direction approximately normal to thesubstrate. The “crystal orientation that is mostly aligned in thedirection approximately normal to the substrate” is not necessarilylimited to crystal orientation that is completely aligned in thedirection normal to the substrate, and means that it may be crystalorientation that is, to some extent, in alignment with the normal or adirection similar thereto as long as desired device properties ofdevices such as light emitting devices including the self-supportingsubstrate can be ensured. Using an expression derived from theproduction method, it can also be said that the gallium nitride-basedsingle crystal grains have a structure in which grains are grown mostlyin conformity with the crystal orientation of an orientedpolycrystalline sintered body used as a base substrate in producing theself-supporting polycrystalline gallium nitride substrate. The“structure in which grains are grown mostly in conformity with thecrystal orientation of an oriented polycrystalline sintered body” meansa structure resulting from crystal growth influenced by the crystalorientation of the oriented polycrystalline sintered body, is notnecessarily limited to a structure in which grains are grown completelyin conformity with the crystal orientation of the orientedpolycrystalline sintered body, and may be a structure in which grainsare grown, to some extent, in conformity with the crystal orientation ofthe oriented polycrystalline sintered body as long as desired deviceproperties of devices such as light emitting devices including theself-supporting substrate can be ensured. That is, this structure alsoincludes a structure in which grains are grown in crystal orientationdifferent from that of the oriented polycrystalline sintered body. Inthis sense, the expression “structure in which grains are grown mostlyin conformity with crystal orientation” can be paraphrased as “structurein which grains are grown in a manner mostly derived from crystalorientation”, and this paraphrasing and the above meaning similarlyapply to similar expressions in this specification. Therefore, suchcrystal growth is preferably epitaxial growth, but it is not limitedthereto, and may take a variety of similar crystal growth forms. In anycase, with crystals grown in this way, the self-supportingpolycrystalline gallium nitride substrate can have a structure, thecrystal orientation of which is mostly aligned with respect to thedirection approximately normal to the substrate.

Inverse pole figure mapping by electron backscatter diffraction (EBSD)analysis performed on the cross-section perpendicular to the substratesurface (plate surface) of the self-supporting polycrystalline galliumnitride substrate can also confirm that the gallium nitride-based singlecrystal grains constituting the self-supporting substrate have aspecific crystal orientation in the direction approximately normal tothe substrate. However, the grains are not oriented in the directionparallel to the plate surface, which is perpendicular to the directionnormal to the substrate. That is, the gallium nitride-based singlecrystal grains are oriented only in the direction approximately normalto the substrate, and the twist (rotation of a crystal axis) of thegallium nitride-based single crystal grains around the axis extending inthe direction approximately normal to the substrate is distributedrandomly. Such a structure enables satisfactory characteristics to beattained in producing devices having light emitting functions anddevices such as solar cells using the self-supporting polycrystallinegallium nitride substrate. Although the reason of this is not clear,this is considered to be attributed to the effect resulting from theeffect of reducing the defect density at the surface of thepolycrystalline gallium nitride substrate and from the light extractionefficiency. Also, the reason of a reduced defect density achieved whenthe twist distribution is random is not clear, but it is considered thatdefects that develop in a tilted manner relative to the direction normalto the substrate disappear at grain boundaries. In other words, it isconsidered that when the gallium nitride-based single crystal grainshave a uniform twist distribution in the horizontal direction, defectsdevelop without disappearing at grain boundaries, thus resulting in alarge defect density at the substrate surface.

Therefore, the self-supporting polycrystalline gallium nitride substrateaccording to the above embodiment is observed as a single crystal whenviewed in the direction normal to the substrate, and it is also possibleto recognize it as an aggregate of gallium nitride-based single crystalgrains having a columnar structure in which grain boundary are observedin a view of the cross section in the horizontal plane direction of thesubstrate. Here, the “columnar structure” does not mean only a typicalvertically long columnar shape, and is defined as having a meaningencompassing various shapes such as a horizontally long shape, atrapezoidal shape, and an inverted trapezoidal shape. As describedabove, the self-supporting polycrystalline gallium nitride substrate mayhave a structure with crystal orientation that is, to some extent, inalignment with the normal or a direction similar thereto, and does notnecessarily need to have a columnar structure in a strict sense. Asdescribed above, the growth of gallium nitride single crystal grains dueto the influence of the crystal orientation of an orientedpolycrystalline sintered body used for production of a self-supportingpolycrystalline gallium nitride substrate is considered to be the causeof the columnar structure. Therefore, the average grain diameter at thecross section (hereinafter referred to as a cross-sectional averagediameter) of gallium nitride single crystal grains that can also becalled columnar structures is considered to depend on not only theconditions of film formation but also the average grain diameter at theplate surface of the oriented polycrystalline sintered body. In the casewhere the self-supporting polycrystalline gallium nitride substrate isused as a part of a light emitting functional layer of a light emittingdevice, the presence of grain boundaries impairs light transmittance inthe cross-sectional direction and causes light to be scattered orreflected. Therefore, in the case of a light emitting device having astructure in which light is extracted in the direction normal to thesubstrate, a luminance increasing effect due to scattered light fromgrain boundaries is also expected.

As described above, in the case where a vertical LED structure is formedusing the self-supporting polycrystalline gallium nitride substrate ofthe present invention, it is preferable that the top surface of theself-supporting substrate on which a light emitting functional layerwill be formed and the bottom surface of the self-supporting substrateon which an electrode will be formed connect without intervention of agrain boundary. That is, it is preferable that the gallium nitride-basedsingle crystal grains exposed at the top surface of the self-supportingpolycrystalline gallium nitride substrate connect to the bottom surfaceof the self-supporting polycrystalline gallium nitride substrate withoutintervention of a grain boundary. The presence of a grain boundarycauses resistance when electricity is applied, and therefore becomes afactor that deteriorates luminous efficiency.

The cross-sectional average diameter D_(T) at the outermost surface ofgallium nitride-based single crystal grains exposed at the top surfaceof the self-supporting polycrystalline gallium nitride substrate ispreferably different from the cross-sectional average diameter D_(B) atthe outermost surface of the gallium nitride-based single crystal grainsexposed at the bottom surface of the self-supporting polycrystallinegallium nitride substrate. In this way, the crystallinities of theself-supporting substrate and its constitutive grains are enhanced. Forexample, when gallium nitride crystals are grown using epitaxial growthvia a vapor phase or a liquid phase, growth occurs not only in thedirection normal to the substrate but also in the horizontal direction,depending on the conditions of film formation. At this time, if thequality of grains that serve as a starting point of growth or of seedcrystals produced thereon varies, the growth rates of respective galliumnitride crystals differ, and, fast-growing grains may grow to coverslow-growing grains. In the case of such a growth behavior, grains onthe top surface side of the substrate are likely to have a largerdiameter than those on the bottom surface side of the substrate. In thiscase, growth of slow-growing crystals terminates halfway, and a grainboundary can be observed also in the direction normal to the substratewhen a certain cross section is observed. However, the grains exposed atthe top surface of the substrate connect to the bottom surface of thesubstrate without intervention of a grain boundary, and there is not aresistive phase against application of an electric current. In otherwords, after gallium nitride crystals are formed into a film, the grainsexposed on the top surface side of the substrate (the side opposite tothe side that was in contact with the base-substrate orientedpolycrystalline sintered body during production) are predominantlygrains that connect to the bottom surface without intervention of agrain boundary, and therefore it is preferable to produce a lightemitting functional layer on the top surface side of the substrate fromthe viewpoint of increasing the luminous efficiency of an LED having avertical structure. On the other hand, on the bottom surface side of thesubstrate (the side that was in contact with the base-substrate orientedpolycrystalline sintered body during production), there are also grainsthat do not connect to the top surface of the substrate, and thus thereis a possibility of impaired luminous efficiency if a light emittingfunctional layer is produced on the bottom surface side of thesubstrate. Moreover, as described above, in the case of such a growthbehavior, grains develop to have a large diameter as they grow, andtherefore, the top surface of the self-supporting polycrystallinegallium nitride substrate can be paraphrased as the side on which thegrain diameter of gallium nitride crystals is larger, and the bottomsurface thereof can be paraphrased as the side on which the graindiameter is smaller. That is, in the self-supporting polycrystallinegallium nitride substrate, it is preferable to produce a light emittingfunctional layer on the side where the grain diameter of gallium nitridecrystals is larger (the top surface side of the substrate) from theviewpoint of increasing the luminous efficiency of an LED having avertical structure. When an oriented polycrystalline alumina sinteredbody that is oriented along the c-plane or the like is used for a basesubstrate, the top surface side of the self-supporting polycrystallinegallium nitride substrate (the side opposite to the side that was incontact with the base-substrate oriented polycrystalline aluminasintered body during production) becomes the gallium surface, and thebottom surface side of the self-supporting polycrystalline galliumnitride substrate (the side that was in contact with the base-substrateoriented polycrystalline alumina sintered body during production)becomes the nitrogen surface. That is, at the gallium surface of theself-supporting polycrystalline gallium nitride substrate, grainsconnecting to the bottom surface without intervention of a grainboundary are predominant. Therefore, it is preferable to produce a lightemitting functional layer on the gallium surface side (the top surfaceside of the substrate) from the viewpoint of increasing the luminousefficiency of an LED having a vertical structure.

Therefore, in the case where grains on the top surface side of thesubstrate exhibit such a growth behavior that their grain diameter islarger than that of grains on the bottom surface side of the substrate,or that is to say, in the case where the cross-sectional averagediameter of gallium nitride-based single crystal grains exposed at thetop surface of the substrate is larger than the cross-sectional averagediameter of gallium nitride-based single crystal grains exposed at thebottom surface of the substrate, luminous efficiency is increased, andtherefore such diameters are preferable (this can be paraphrased that itis preferable that the number of gallium nitride-based single crystalgrains exposed at the top surface of the substrate is smaller than thenumber of gallium nitride-based single crystal grains exposed at thebottom surface of the substrate). Specifically, the ratio D_(T)/D_(B),which is the ratio of the cross-sectional average diameter at theoutermost surface of gallium nitride-based single crystal grains exposedat the top surface of the self-supporting polycrystalline galliumnitride substrate (hereinafter referred to as the cross-sectionalaverage diameter D_(T) at the top surface of the substrate) to thecross-sectional average diameter at the outermost surface of galliumnitride-based single crystal grains exposed at the bottom surface of theself-supporting polycrystalline gallium nitride substrate (hereinafterreferred to as the cross-sectional average diameter D_(B) at the bottomsurface of the substrate), is preferably greater than 1.0, morepreferably 1.5 or greater, even more preferably 2.0 or greater,particularly preferably 3.0 or greater, and most preferably 5.0 orgreater. However, an excessively high D_(T)/D_(B) ratio may in turnresult in impaired luminous efficiency, and therefore a ratio of 20 orless is preferable, and 10 or less is more preferable. Although thereason of change in luminous efficiency is not clear, it is consideredthat when the ratio D_(T)/D_(B) is high, the area of grain boundariesthat do not contribute to light emission is reduced due to the increasedgrain diameter, or crystal defects are reduced due to the increasedgrain diameter. Although the reason of reduction in crystal defect isnot clear either, it is also considered that defective grains growslowly, and grains with less defects grow fast. On the other hand, whenthe ratio D_(T)/D_(B) is excessively high, the cross-sectional diameterof grains that connect between the substrate top surface and thesubstrate bottom surface (i.e., grains exposed at the top surface sideof the substrate) is small near the bottom surface side of thesubstrate. As a result, sufficient electric current paths are notobtained, which can be considered as a cause of reduction in luminousefficiency, but the details thereof are not clear.

Crystallinity at the interface between columnar structures constitutingthe self-supporting polycrystalline gallium nitride substrate is low,and therefore when the self-supporting polycrystalline gallium nitridesubstrate is used as a light emitting functional layer of a lightemitting device, there is a possibility that the luminous efficiencydeteriorates, the emission wavelength changes, and the emissionwavelength broadens. Therefore, a larger cross-sectional averagediameter of the columnar structures is preferable. Preferably, thecross-sectional average diameter of gallium nitride-based single crystalgrains at the outermost surface of the self-supporting polycrystallinegallium nitride substrate is 0.3 μm or greater, more preferably 3 μm orgreater, even more preferably 10 μm or greater, yet more preferably 20μm or greater, particularly preferably 50 μm or greater, particularlymore preferably 70 μm or greater, and most preferably 100 μm or greater.Although the upper limit of the cross-sectional average diameter of thegallium nitride-based single crystal grains at the outermost surface ofthe self-supporting polycrystalline gallium nitride substrate is notparticularly limited, it is realistically 1000 μm or less, morerealistically 500 μm or less, and even more realistically 200 μm orless. In order to produce gallium nitride-based single crystal grainshaving such a cross-sectional average diameter, it is desirable that thesintered grain diameter at the plate surface of grains that constitutethe oriented polycrystalline sintered body used for producing theself-supporting polycrystalline gallium nitride substrate is 0.3 μm to1000 μm, more desirably 3 μm to 1000 μm, even more desirably 10 μm to800 μm, and particularly desirably 14 μm to 500 μm. Alternatively, witha view to making the cross-sectional average diameter of galliumnitride-based single crystal grains at the outermost surface of theself-supporting polycrystalline gallium nitride substrate larger thanthe cross-sectional average diameter at the bottom surface of theself-supporting substrate, it is desirable that the sintered graindiameter at the plate surface of grains constituting the orientedpolycrystalline sintered body is 10 μm to 100 μm and more desirably 14μm to 70 μm.

The gallium nitride-based single crystal grains constituting theself-supporting polycrystalline gallium nitride substrate may be freefrom a dopant. Here, the phrase “free from a dopant” means that anelement that is added to impart a certain function or property is notcontained, but, needless to say, inevitable impurities are allowed.Alternatively, the gallium nitride-based single crystal grainsconstituting the self-supporting polycrystalline gallium nitridesubstrate may be doped with an n-type dopant or a p-type dopant, and, inthis case, the self-supporting polycrystalline gallium nitride substratecan be used as a component or a layer other than a substrate, such as ap-type electrode, an n-type electrode, a p-type layer, or an n-typelayer. Preferable examples of p-type dopants include one or moreselected from the group consisting of beryllium (Be), magnesium (Mg),calcium (Ca), strontium (Sr), zinc (Zn), and cadmium (Cd). Preferableexamples of n-type dopants include one or more selected from the groupconsisting of silicon (Si), germanium (Ge), tin (Sn), and oxygen (O).

The gallium nitride-based single crystal grains constituting theself-supporting polycrystalline gallium nitride substrate may be formedinto mixed crystals for band gap control. Preferably, the galliumnitride single crystal grains may be composed of gallium nitride formedinto mixed crystals with crystals of one or more selected from the groupconsisting of AlN and InN, and p-type gallium nitride and/or n-typegallium nitride single crystal grains may be those in which suchmixed-crystal gallium nitride is doped with a p-type dopant or an n-typedopant. For example, doping Al_(x)Ga_(1-x)N, which is a mixed crystal ofgallium nitride and AlN, with Mg makes it possible to provide a p-typesubstrate, and doping Al_(x)Ga_(1-x)N with Si makes it possible toprovide an n-type substrate. When the self-supporting substrate is usedas a light emitting functional layer of a light emitting device, forminggallium nitride into a mixed crystal with AlN widens the band gap andmakes it possible to shift the emission wavelength toward the highenergy side. Moreover, gallium nitride may be formed into a mixedcrystal with InN, and this narrows the band gap and makes it possible toshift the emission wavelength toward the low energy side.

It is preferable that the self-supporting polycrystalline galliumnitride substrate has a size of 50.8 mm (2 inches) or greater indiameter, more preferably 100 mm (4 inches) or greater in diameter, andeven more preferably 200 mm (8 inches) or greater in diameter. Thelarger the self-supporting polycrystalline gallium nitride substrate is,the greater the number of producible devices is, and therefore a largersize is preferable from the viewpoint of production cost, and is alsopreferable from the viewpoint of use in surface-light-emitting devicesbecause the usable device area is enlarged so as to expand applicationsto surface-emitting lightings and the like. Therefore, the upper limitsof the area and size thereof should not be specified. It is preferablethat the self-supporting polycrystalline gallium nitride substrate iscircular or substantially circular as viewed from above, but the shapeis not limited thereto. In the case where the self-supporting galliumnitride substrate is not circular or substantially circular, the area ispreferably 2026 mm² or greater, more preferably 7850 mm² or greater, andeven more preferably 31400 mm² or greater. For applications that do notrequire a large area, the area may be smaller than the above range suchas 50.8 mm (2 inches) or less in diameter, or 2026 mm² or less in termsof area. The thickness of the self-supporting polycrystalline galliumnitride substrate needs to be capable of imparting self-supportingproperties to the substrate, and is thus preferably 20 μm or greater,more preferably 100 μm or greater, and even more preferably 300 μm orgreater. Although the upper limit of the thickness of theself-supporting polycrystalline gallium nitride substrate should not bespecified, the thickness is realistically 3000 μm or less from theviewpoint of production cost.

The aspect ratio T/D_(T), which is defined as the ratio of the thicknessT of the self-supporting polycrystalline gallium nitride substrate tothe cross-sectional average diameter D_(T) at the outermost surface ofgallium nitride-based single crystal grains exposed at the top surfaceof the self-supporting polycrystalline gallium nitride substrate, ispreferably 0.7 or greater, more preferably 1.0 or greater, and even morepreferably 3.0 or greater. This aspect ratio is preferable from theviewpoint of increasing luminous efficiency in the case of LEDs. As forthe cause of increased luminous efficiency, it is considered that grainswith a high aspect ratio result in a low defect density in galliumnitride, increased light extraction efficiency, and so on, but detailsthereof are not clear.

As described so far, from the viewpoint of increasing luminousefficiency, it is preferable that (1) a light emitting functional layeris produced on the top surface side of the self-supporting substrate(the side opposite to the side that was in contact with thebase-substrate oriented polycrystalline sintered body duringproduction), (2) the ratio D_(T)/D_(B), which is the ratio of thecross-sectional average diameter D_(T) of the substrate top surface tothe cross-sectional average diameter D_(B) of the substrate bottomsurface, is at a suitable value, (3) the cross-sectional averagediameter at the substrate outermost surface of grains constituting theself-supporting substrate is large, and (4) the aspect ratio T/D_(T) ofgrains constituting the self-supporting substrate is large. From theviewpoints (3) and (4) above, it is preferable that the cross-sectionalaverage diameter is large and the aspect ratio is large, or in otherwords, a gallium nitride crystal that has a large cross-sectionalaverage diameter on the top surface side of the substrate and a largethickness is preferable. Moreover, from the self-supporting viewpoint,the thickness of the self-supporting polycrystalline gallium nitridesubstrate is preferably 20 μm or greater, more preferably 100 μm orgreater, and even more preferably 300 μm or greater. However, asdescribed above, a large thickness of a gallium nitride crystal is notpreferable from the cost viewpoint, and as long as the substrate isself-supporting, a lower thickness is preferable. That is, the thicknessof the self-supporting polycrystalline gallium nitride substrate isrealistically 3000 μm or less, preferably 600 μm or less, and preferably300 μm or less. Therefore, the thickness is preferably about 50 to 500μm and more preferably about 50 to 300 μm from the viewpoint of allowingthe substrate to be self-supporting and increasing the luminousefficiency as well as from the viewpoint of cost.

Manufacturing Method

The self-supporting polycrystalline gallium nitride substrate of thepresent invention can be manufactured by (1) providing an orientedpolycrystalline sintered body, (2) forming a seed crystal layer composedof gallium nitride on the oriented polycrystalline sintered body so thatthe seed crystal layer has crystal orientation mostly in conformity withthe crystal orientation of the oriented polycrystalline sintered body,(3) forming a layer with a thickness of 20 μm or greater composed ofgallium nitride-based crystals on the seed crystal layer so that thelayer has crystal orientation mostly in conformity with the crystalorientation of the seed crystal layer, and (4) removing the orientedpolycrystalline sintered body to obtain the self-supportingpolycrystalline gallium nitride substrate.

(1) Oriented Polycrystalline Sintered Body

An oriented polycrystalline sintered body is provided as a basesubstrate for producing a self-supporting polycrystalline galliumnitride substrate. Although the composition of the orientedpolycrystalline sintered body is not particularly limited, the orientedpolycrystalline sintered body is preferably one selected from anoriented polycrystalline alumina sintered body, an orientedpolycrystalline zinc oxide sintered body, and an orientedpolycrystalline aluminum nitride sintered body. The orientedpolycrystalline sintered body can be efficiently manufactured throughforming and firing using a commercially available plate-shaped powder,and thus is not only able to be produced at low cost but also suitablefor having a large area due to ease in forming. By using an orientedpolycrystalline sintered body as a base substrate and allowing aplurality of semiconductor single crystal grains to grow thereon, it ispossible to manufacture a self-supporting polycrystalline galliumnitride substrate that is suitable for manufacturing large-area lightemitting devices at low cost. As a result, the self-supportingpolycrystalline gallium nitride substrate is extremely suitable formanufacturing large-area light emitting devices at low cost.

The oriented polycrystalline sintered body is composed of a sinteredbody that contains numerous single crystal grains which are to someextent or highly oriented in a certain direction. The use of apolycrystalline sintered body oriented in this way makes it possible toproduce a self-supporting polycrystalline gallium nitride substratehaving crystal orientation that is mostly aligned in the directionapproximately normal to the substrate, and when a gallium nitride-basedmaterial is formed on the self-supporting polycrystalline galliumnitride substrate by epitaxial growth or crystal growth similar thereto,a state in which crystal orientation is mostly aligned in the directionapproximately normal to the substrate is achieved. Accordingly, the useof such a highly oriented self-supporting polycrystalline galliumnitride substrate as a substrate for a light emitting device makes itpossible to form a light emitting functional layer that is similarly ina state in which its crystal orientation is mostly aligned in thedirection approximately normal to the substrate and makes it possible toachieve high luminous efficiency identical to that obtained when asingle crystal substrate is used. Alternatively, even when this highlyoriented self-supporting polycrystalline gallium nitride substrate isused as a light emitting functional layer of a light emitting device, itis possible to achieve high luminous efficiency identical to thatobtained when a single crystal substrate is used. In any case, in orderto produce such a highly oriented self-supporting polycrystallinegallium nitride substrate, an oriented polycrystalline sintered bodyneeds to be used as a base substrate. Although it is preferable that theoriented polycrystalline sintered body is transparent or translucent,the sintered body is not limited in this respect. In the case where thesintered body is transparent or translucent, a technique such as laserlift-off can be used for removing the oriented polycrystalline plate. Inaddition to commonly used pressureless sintering methods using an airatmosphere furnace, a nitrogen atmosphere furnace, a hydrogen atmospherefurnace, or the like, pressure sintering methods such as hot isostaticpressing (HIP), hot pressing (HP), and spark plasma sintering (SPS), andcombination thereof can be used as production methods for obtaining theoriented polycrystalline sintered body.

The oriented polycrystalline sintered body preferably has a size of 50.8mm (2 inches) or greater in diameter, more preferably 100 mm (4 inches)or greater in diameter, and even more preferably 200 mm (8 inches) orgreater in diameter. The larger the oriented polycrystalline sinteredbody is, the larger the area of the producible self-supportingpolycrystalline gallium nitride substrate is and thus the more thenumber of producible light emitting devices is, and therefore a largersize is preferable from the viewpoint of production cost. Moreover, alarger size is also preferable from the viewpoint of use insurface-light-emitting devices because the usable device area isenlarged so as to expand applications to surface-emitting lightings andthe like, and therefore, the upper limits of the area and size thereofshould not be specified. It is preferable that the self-supportingpolycrystalline gallium nitride substrate is circular or substantiallycircular as viewed from above, but the shape is not limited thereto. Inthe case where the self-supporting polycrystalline gallium nitridesubstrate is not circular or substantially circular, the area ispreferably 2026 mm² or greater, more preferably 7850 mm² or greater, andeven more preferably 31400 mm² or greater. For applications that do notrequire a large area, the area may be smaller than the above range suchas 50.8 mm (2 inches) or less in diameter, or 2026 mm² or less in termsof area. Although the thickness of the oriented polycrystalline sinteredbody is not limited as long as it is self-supporting, an excessivelylarge thickness is not preferable from the viewpoint of production cost.Therefore, the thickness is preferably 20 μm or greater, more preferably100 μm or greater, and even more preferably 100 to 1000 μm. Meanwhile,in the case of forming gallium nitride into a film, the entire substratemay warp due to stress resulting from the difference between the thermalexpansions of alumina and gallium nitride, thus adversely affecting thesubsequent process. Although stress varies according to, for example,the method for forming a gallium nitride film, film formationconditions, materials of the oriented polycrystalline sintered body,film thickness, and substrate diameter, a thick oriented polycrystallinesintered body may be used as a base substrate as one technique ofsuppressing warpage due to stress. For example, in the case of producinga self-supporting polycrystalline gallium nitride substrate having adiameter of 50.8 mm (2 inches) and a thickness of 300 μm using anoriented polycrystalline alumina sintered body as a base orientedpolycrystalline sintered body, the thickness of the orientedpolycrystalline alumina sintered body may be 900 μm or greater, 1300 μmor greater, or 2000 μm or greater. In this way, the thickness of theoriented polycrystalline sintered body may be suitably selected inconsideration of, for example, the viewpoint of production cost and theviewpoint of suppressing warpage.

The average grain diameter at the plate surface of grains constitutingthe oriented polycrystalline sintered body is preferably 0.3 to 1000 μm,more preferably 3 to 1000 μm, even more preferably 10 μm to 200 μm, andparticularly preferably 14 μm to 200 μm. Alternatively, as describedabove, in the case of considering that the cross-sectional averagediameter of semiconductor single crystal grains at the outermost surfaceof the self-supporting polycrystalline gallium nitride substrate is madelarger than the cross-sectional average diameter at the bottom surfaceof the self-supporting substrate, the sintered grain diameter at theplate surface of grains constituting the oriented polycrystallinesintered body is preferably 10 μm to 100 μm and more preferably 14 μm to70 μm. The overall average grain diameter of the orientedpolycrystalline sintered body correlates with the average grain diameterat the plate surface, and when the diameter is within these ranges, thesintered body is excellent in terms of mechanical strength and is easyto be handled. Moreover, when a light emitting device is produced byforming a light emitting functional layer in the upper part and/or theinterior of a self-supporting polycrystalline gallium nitride substrateproduced using an oriented polycrystalline sintered body, the luminousefficiency of the light emitting functional layer is also excellent. Theaverage grain diameter at the plate surface of sintered body grains inthe present invention is measured by the following method. That is, theplate surface of a plate-shaped sintered body is polished, and an imageis taken with a scanning electron microscope. The visual field range isdetermined in a way such that when straight lines are diagonally drawnon the obtained image, each straight line crosses 10 to 30 grains. Theaverage grain diameter at the plate surface is determined by diagonallydrawing two straight lines on the obtained image, taking the average ofthe line segment lengths inside all grains crossed by the straightlines, and multiplying the average by 1.5. When the boundary of sinteredbody grains cannot be clearly determined on the scanning microscopeimage of the plate surface, the above evaluation may be carried outafter performing processing to emphasize the boundary by thermal etching(for example, for 45 minutes at 1550° C.) or chemical etching.

A particularly preferable oriented polycrystalline sintered body is anoriented polycrystalline alumina sintered body. Alumina is aluminumoxide (Al₂O₃) and is typically α-alumina having the same corundum-typestructure as single crystal sapphire, and the oriented polycrystallinealumina sintered body is a solid in which a countless number of aluminacrystal grains in an oriented state are bonded to each other bysintering. Alumina crystal grains contain alumina and may contain adopant and an inevitable impurity as other elements, or that may becomposed of alumina and an inevitable impurity. The orientedpolycrystalline alumina sintered body may contain an additive as asintering aid in a grain boundary phase. Although the orientedpolycrystalline alumina sintered body may also contain another phase oranother element as described above in addition to alumina crystalgrains, preferably the oriented polycrystalline alumina sintered body iscomposed of alumina crystal grains and an inevitable impurity. Theoriented plane of the oriented polycrystalline alumina sintered body isnot particularly limited and may be a c-plane, an a-plane, an r-plane,an m-plane, or the like.

The direction in which crystals are oriented in the orientedpolycrystalline alumina sintered body is not particularly limited, andit may be the direction of a c-plane, an a-plane, an r-plane, anm-plane, or the like, and from the viewpoint of lattice constantmatching with the self-supporting polycrystalline gallium nitridesubstrate, it is preferable that crystals are oriented along thec-plane. As for the degree of orientation, for example, the degree oforientation at the plate surface is preferably 50% or greater, morepreferably 65% or greater, even more preferably 75% or greater,particularly preferably 85% or greater, particularly more preferably 90%or greater, and most preferably 95% or greater. The degree oforientation can be determined by measuring an XRD profile throughirradiating the plate surface of plate-shaped alumina with X rays usingan XRD apparatus (such as RINT-TTR III manufactured by RigakuCorporation) and calculating according to the formulae below.

$\begin{matrix}{{{{Degree}\mspace{14mu}{of}\mspace{14mu}{{Orientation}\mspace{11mu}\lbrack\%\rbrack}} = {\frac{p - p_{0}}{1 - p_{0}} \times 100}}{p_{0} = \frac{I_{0}({hkl})}{\sum{I_{0}({hkl})}}}{p = \frac{I_{s}({hkl})}{\sum{I_{s}({hkl})}}}} & \left\lbrack {{Mathematical}\mspace{14mu}{Formula}\mspace{20mu} 1} \right\rbrack\end{matrix}$where I₀(hkl) and I_(s)(hkl) are the integral values)(2θ=20-70° of thediffraction intensities from the (hkl) planes in ICDD No. 461212 and asample, respectively.

As described above, the crystallinity of constitutive grains of theself-supporting polycrystalline gallium nitride substrate of the presentinvention tends to be high, and the density of defects such asdislocation can be kept low. Accordingly, it is considered that, in someapplications such as light emitting devices, it is even possible to usethe self-supporting polycrystalline gallium nitride substrate morepreferably than a gallium nitride single crystal substrate. For example,when a functional layer is produced on the self-supportingpolycrystalline gallium nitride substrate by epitaxial growth, thefunctional layer grows mostly in conformity with the baseself-supporting polycrystalline gallium nitride substrate and becomes anaggregate of columnar structures. In epitaxial growth, the crystalquality of the base is inherited, and therefore it is possible to obtainhigh crystal quality in each domain of columnar structures constitutingthe functional layer. Although the reason why the crystal grainsconstituting the self-supporting polycrystalline gallium nitridesubstrate has a low defect density is not clear, it is presumed thatamong the lattice defects occurring during the initial stage ofproduction of the self-supporting polycrystalline gallium nitridesubstrate, those that develop with tilt toward the horizontal directionare absorbed by the grain boundary as growth progresses, and thusdisappear.

The oriented polycrystalline alumina sintered body can be manufacturedby forming and sintering, using a plate-shaped alumina powder as a rawmaterial. A plate-shaped alumina powder is sold in the market and iscommercially available. Although the type and the shape of theplate-shaped alumina powder are not particularly limited as long as adense oriented polycrystalline alumina sintered body can be obtained,the average grain diameter may be 0.4 to 15 μm and the thickness may be0.05 to 1 μm, and a mixture of two or more raw materials havingdifferent average grain diameters within this range may be used.Preferably, a plate-shaped alumina powder can be formed into an orientedgreen body by a technique that uses shearing force. Preferable examplesof techniques that use shearing force include tape casting, extrusionmolding, doctor blade method, and any combination of these. In theorientation technique that uses shearing force, it is preferable, in anytechnique exemplified above, that additives such as a binder, aplasticizer, a dispersing agent, and a dispersion medium are suitablyadded to the plate-shaped alumina powder to form a slurry, and theslurry is allowed to pass through a slit-shaped narrow discharge port todischarge and form the slurry into a sheet on a base. The slit width ofthe discharge port is preferably 10 to 400 μm. The amount of thedispersion medium is adjusted so that the viscosity of the slurry ispreferably 5000 to 100000 cP and more preferably 20000 to 60000 cP. Thethickness of the oriented green body formed into a sheet is preferably 5to 500 μm and more preferably 10 to 200 μm. It is preferable thatnumerous pieces of this oriented green body that has been formed into asheet are stacked on top of the other to form a precursor laminatehaving a desired thickness, and press molding is performed on thisprecursor laminate. This press molding can be preferably performed bypacking the precursor laminate in a vacuum pack or the like andsubjecting it to isostatic pressing in hot water at 50 to 95° C. under apressure of 10 to 2000 kgf/cm². Moreover, the oriented green body thathas been formed into a sheet, or the precursor laminate, may besubjected to processing by a roll press method (such as a heating rollpress or a calender roll). Moreover, when extrusion molding is used, theflow channel within a metal mold may be designed so that after passingthrough a narrow discharge port within the metal mold, sheets of thegreen body are integrated into a single body within the metal mold, andthe green body is ejected in a laminated state. It is preferable todegrease the resulting green body in accordance with known conditions.The oriented green body obtained in the above manner is fired by, inaddition to ordinal pressureless firing using an air atmosphere furnace,a nitrogen atmosphere furnace, a hydrogen atmosphere furnace, or thelike, pressure sintering methods such as hot isostatic pressing (HIP),hot pressing (HP), and spark plasma sintering (SPS), and combinationthereof, to form an alumina sintered body containing oriented aluminacrystal grains. Although the firing temperature and the firing time inthe above firing depend on the firing method, the firing temperature maybe 1000 to 1950° C., preferably 1100 to 1900° C., and more preferably1500 to 1800° C., and the firing time may be 1 minute to 10 hours andpreferably 30 minutes to 5 hours. Form the viewpoint of promotingdensification, firing is preferably performed through a first firingstep of firing the green body with a hot press at 1500 to 1800° C. for 2to 5 hours under a surface pressure of 100 to 200 kgf/cm², and a secondfiring step of re-firing the resulting sintered body with a hotisostatic press (HIP) at 1500 to 1800° C. for 30 minutes to 5 hoursunder a gas pressure of 1000 to 2000 kgf/cm². Although the firing timeat the aforementioned firing temperature is not particularly limited, itis preferably 1 to 10 hours and more preferably 2 to 5 hours. In thecase of imparting translucency, a preferable example is a method inwhich a high-purity plate-shaped alumina powder is used as a rawmaterial and fired in an air atmosphere furnace, a hydrogen atmospherefurnace, a nitrogen atmosphere furnace, or the like at 1100 to 1800° C.for 1 minute to 10 hours. A method may be used in which the resultingsintered body is re-fired in a hot isostatic press (HIP) at 1200 to1400° C. or 1400 to 1950° C. for 30 minutes to 5 hours under a gaspressure of 300 to 2000 kgf/cm². The fewer the grain boundary phasesare, the more preferable it is, and therefore it is preferable that theplate-shaped alumina powder has high purity, and more preferably thepurity is 98% or higher, even more preferably 99% or higher,particularly preferably 99.9% or higher, and most preferably 99.99% orhigher. The firing conditions are not limited to those described above,and the second firing step with, for example, hot isostatic pressing(HIP) may be omitted as long as densification and high orientation canbe simultaneously achieved. Moreover, an extremely small amount ofadditive may be added to the raw material as a sintering aid. Additionof a sintering aid, although it is contradictory to reducing the amountof grain boundary phase, is for reducing pores that are one of thefactors causing scattering of light and, as a result, increasingtranslucency. Examples of such sintering aids include at least oneselected from oxides such as MgO, ZrO₂, Y₂O₃, CaO, SiO₂, TiO₂, Fe₂O₃,Mn₂O₃, and La₂O₃, fluorides such as AlF₃, MgF₂, and YbF₃, and the like.Among these, MgO, CaO, SiO₂, and La₂O₃ are preferable, and MgO isparticularly preferable. However, from the translucency viewpoint, theamount of additive should be limited to the smallest necessary level,and is preferably 5000 ppm or less, more preferably 1000 ppm or less,and even more preferably 700 ppm or less.

Moreover, the oriented polycrystalline alumina sintered body can beproduced also by forming and sintering, using a mixed powder in which aplate-shaped alumina powder is suitably added to a fine alumina powderand/or transition alumina powder as a raw material. In this productionmethod, crystal growth and densification occur through a so-called TGG(Templated Grain Growth) process in which the plate-shaped aluminapowder serves as a seed crystal (template), the fine alumina powderand/or transition alumina powder serves as a matrix, and the templategrows homoepitaxially while incorporating the matrix. As for the graindiameters of the plate-shaped alumina grains serving as a template andof the matrix, the larger the grain diameter ratio thereof is, the moreeasily the grains grow. For example, when the average grain diameter ofthe template is 0.5 to 15 μm, the average grain diameter of the matrixis preferably 0.4 μm or less, more preferably 0.2 μm or less, and evenmore preferably 0.1 μm or less. Although the mixing ratio of thetemplate and the matrix varies according to the grain diameter ratio,firing conditions, and presence or absence of an additive, thetemplate/matrix ratio may be 50/50 to 1/99 wt % when, for example, aplate-shaped alumina powder having an average grain diameter of 2 μm isused for the template and a fine alumina powder having an average graindiameter of 0.1 μm is used for the matrix. From the viewpoint ofpromoting densification, at least one selected from oxides such as MgO,ZrO₂, Y₂O₃, CaO, SiO₂, TiO₂, Fe₂O₃, Mn₂O₃, and La₂O₃, fluorides such asAlF₃, MgF₂, and YbF₃, and the like may be added as a sintering aid, andMgO, CaO, SiO₂, and La₂O₃ are preferable, and MgO is particularlypreferable. In such a technique as well, a high-quality orientedpolycrystalline alumina sintered body can be obtained by theaforementioned pressure sintering methods such as hot isostatic pressing(HIP), hot pressing (HP), and spark plasma sintering (SPS), andcombination thereof, in addition to ordinal pressureless firing using anair atmosphere furnace, a nitrogen atmosphere furnace, a hydrogenatmosphere furnace, or the like.

The alumina sintered body obtained in this way is a polycrystallinealumina sintered body oriented in the direction of a desired plane suchas a c-plane in accordance with the type of the aforementionedraw-material plate-shaped alumina powder. It is preferable that theoriented polycrystalline alumina sintered body obtained in this way isground with grinding wheel to flatten the plate surface, and then theplate surface is smoothed by lapping using diamond abrasive grains toobtain an oriented alumina substrate.

(2) Formation of Seed Crystal Layer

A seed crystal layer composed of gallium nitride is formed on theoriented polycrystalline sintered body so that the seed crystal layerhas crystal orientation that is mostly in conformity with the crystalorientation of the oriented polycrystalline sintered body. The phrase“formed so that the seed crystal layer has crystal orientation that ismostly in conformity with the crystal orientation of the orientedpolycrystalline sintered body” means a structure resulting from crystalgrowth influenced by the crystal orientation of the orientedpolycrystalline sintered body, and it is not necessarily limited to astructure in which grains are grown completely in conformity with thecrystal orientation of the oriented polycrystalline sintered body andalso includes a structure in which grains grow in a crystal orientationdifferent from that of the oriented polycrystalline sintered body. Amethod for producing a seed crystal layer is not particularly limited,and preferable examples include vapor phase methods such as MOCVD (metalorganic chemical vapor deposition), MBE (molecular beam epitaxy), HVPE(halide vapor phase epitaxy), and sputtering, liquid phase methods suchas Na flux method, ammonothermal method, hydrothermal method, andsol-gel method, powder methods that utilize solid phase growth ofpowder, and combinations of these. For example, formation of a seedcrystal layer by MOCVD is preferably performed by depositing a 20-50 nmthick low-temperature GaN layer at 450 to 550° C. and laminating a GaNfilm having a thickness of 2 to 4 μm at 1000 to 1200° C.

(3) Formation of Gallium Nitride-Based Crystal Layer

A layer with a thickness of 20 μm or greater composed of galliumnitride-based crystals is formed on the seed crystal layer so that thelayer has crystal orientation that is mostly in conformity with thecrystal orientation of the seed crystal layer. A method for forming alayer composed of gallium nitride-based crystals is not particularlylimited as long as the layer has crystal orientation that is mostly inconformity with the crystal orientation of the oriented polycrystallinesintered body and/or the seed crystal layer. Preferable examples includevapor phase methods such as MOCVD and HVPE, liquid phase methods such asNa flux method, ammonothermal method, hydrothermal method, and sol-gelmethod, powder methods that utilize solid phase growth of powder, andcombinations of these, and it is particularly preferable that the layeris formed by Na flux method. A highly crystalline, thick gallium nitridecrystal layer can be efficiently produced on the seed crystal layer byNa flux method. It is preferable that formation of a galliumnitride-based crystal layer by Na flux method is performed by filling acrucible containing a seed crystal substrate with a melt compositioncontaining metal Ga and metal Na and optionally a dopant (e.g., ann-type dopant such as germanium (Ge), silicon (Si), or oxygen (O), or ap-type dopant such as beryllium (Be), magnesium (Mg), calcium (Ca),strontium (Sr), zinc (Zn), or cadmium (Cd)), increasing the temperatureand the pressure to 830 to 910° C. and 3.5 to 4.5 MPa in a nitrogenatmosphere, and then rotating the crucible while retaining thetemperature and the pressure. Although the retention time depends on theintended film thickness, it may be about 10 to 100 hours. Moreover, itis preferable that the gallium nitride crystals obtained by Na fluxmethod are ground with grinding wheel to flatten the plate surface, andthen the plate surface is smoothed by lapping using diamond abrasivegrains.

(4) Removal of Oriented Polycrystalline Sintered Body

The self-supporting polycrystalline gallium nitride substrate can beobtained by removing the oriented polycrystalline sintered body. Amethod for removing the oriented polycrystalline sintered body is notparticularly limited, and examples include grinding, chemical etching,interfacial heating by laser irradiation from the oriented sintered bodyside (laser lift-off), spontaneous separation utilizing a difference inthermal expansion induced by the temperature increase, and the like.

Light Emitting Device and Manufacturing Method Therefor

A high-quality light emitting device can be produced using theself-supporting polycrystalline gallium nitride substrate of the presentinvention described above. As described above, forming a light emittingdevice from the self-supporting polycrystalline gallium nitridesubstrate of the present invention in which the constitutive grains havea tilted crystal orientation makes it possible to attain a higherluminous efficiency than a light emitting device formed from aself-supporting polycrystalline gallium nitride substrate in which theconstitutive grains do not have a tilted crystal orientation. Neitherthe structure of the light emitting device including the self-supportingpolycrystalline gallium nitride substrate of the present invention northe production method therefor is particularly limited. Typically, it ispreferable that the light emitting device is produced by providing alight emitting functional layer on the self-supporting polycrystallinegallium nitride substrate, and formation of this light emittingfunctional layer is performed by forming at least one layer composed ofa plurality of semiconductor single crystal grains, wherein the at leastone layer has a single crystal structure in the direction approximatelynormal to the substrate so that the at least one layer has crystalorientation mostly in conformity with the crystal orientation of thegallium nitride substrate. The self-supporting polycrystalline galliumnitride substrate may be used as a component or a layer other than abase material, such as an electrode (which may be a p-type electrode oran n-type electrode), a p-type layer, an n-type layer, or the like, toproduce a light emitting device. The device size is not particularlylimited, and the device may be a small device having no greater than 5mm×5 mm or may be a surface-emitting device having no less than 10 cm×10cm.

FIG. 1 schematically shows the layer structure of a light emittingdevice according to one embodiment of the present invention. A lightemitting device 10 shown in FIG. 1 includes a self-supportingpolycrystalline gallium nitride substrate 12 and a light emittingfunctional layer 14 formed on this substrate. The light emittingfunctional layer 14 has at least one layer composed of a plurality ofsemiconductor single crystal grains, wherein the at least one layer hasa single crystal structure in the direction approximately normal to thesubstrate. This light emitting functional layer 14 emits light based onthe principle of light emitting devices such as LEDs by suitablyproviding electrodes and the like and applying voltage. In particular,by using the self-supporting polycrystalline gallium nitride substrate12 of the present invention, it can also be expected to obtain a lightemitting device having luminous efficiency equivalent to that when agallium nitride single crystal substrate is used, and a significant costreduction can be achieved. Moreover, by using a gallium nitridesubstrate provided with electroconductivity by introducing a p-type orn-type dopant, it is possible to achieve a light emitting device havinga vertical structure and, thereby, an increased luminance. In addition,a large-area surface emitting device can be achieved at low cost.

The light emitting functional layer 14 is formed on the substrate 12.The light emitting functional layer 14 may be formed on the entiresurface or a part of the substrate 12, or when a buffer layer asdescribed later is formed on the substrate 12, the light emittingfunctional layer 14 may be formed on the entire surface or a part of thebuffer layer in the case. The light emitting functional layer 14 has atleast one layer composed of a plurality of semiconductor single crystalgrains wherein the at least one layer has a single crystal structure inthe direction approximately normal to the substrate, and can take avariety of known layer configurations that bring about light emissionbased on the principle of light emitting devices represented by LEDs bysuitably providing an electrode and/or a phosphor and applying voltage.Therefore, the light emitting functional layer 14 may emit visible lightsuch as blue or red, or may emit ultraviolet light with or withoutvisible light. It is preferable that the light emitting functional layer14 constitutes at least part of a light emitting device that utilizes ap-n junction, and this p-n junction may include an active layer 14 bbetween a p-type layer 14 a and an n-type layer 14 c as shown in FIG. 1.At this time, a double heterojunction or a single heterojunction(hereinafter collectively referred to as a heterojunction) in which alayer having a smaller band gap than the p-type layer and/or the n-typelayer is used as the active layer may be used. Moreover, as one form ofp-type layer/active layer/n-type layer, a quantum well structure inwhich the thickness of the active layer is made small can be adopted.Needless to say, in order to obtain a quantum well, a doubleheterojunction should be employed in which the band gap of the activelayer is made smaller than those of the p-type layer and the n-typelayer. Moreover, a multiple quantum well structure (MQW) may be used inwhich a large number of such quantum well structures are stacked.Adopting these structures makes it possible to increase luminousefficiency in comparison to a p-n junction. In this way, it ispreferable that the light emitting functional layer 14 includes a p-njunction and/or a heterojunction and/or a quantum well junction having alight emitting function.

Therefore, the at least one layer constituting the light emittingfunctional layer 14 can include at least one selected from the groupconsisting of an n-type layer doped with an n-type dopant, a p-typelayer doped with a p-type dopant, and an active layer. The n-type layer,the p-type layer, and the active layer (if present) may be composed ofmaterials whose main components are the same or different to each other.

The material of each layer constituting the light emitting functionallayer 14 is not particularly limited as long as it grows mostly inconformity with the crystal orientation of the self-supportingpolycrystalline gallium nitride substrate and has a light emittingfunction, and is preferably composed of a material whose main componentis at least one selected from gallium nitride (GaN)-based materials,zinc oxide (ZnO)-based materials, and aluminum nitride (AlN)-basedmaterials, and may suitably contain a dopant for controlling it to be ap-type or an n-type. A particularly preferable material is a galliumnitride (GaN)-based material, which is the same type of material as theself-supporting polycrystalline gallium nitride substrate. Moreover, thematerial constituting the light emitting functional layer 14 may be amixed crystal in which AlN, InN, or the like forms a solid solution withGaN, for controlling the band gap thereof. Moreover, as described in thelast paragraph, the light emitting functional layer 14 may be aheterojunction composed of multiple types of material systems. Forexample, a gallium nitride (GaN)-based material may be used for thep-type layer, and a zinc oxide (ZnO)-based material may be used for then-type layer. Moreover, a zinc oxide (ZnO)-based material may be usedfor the p-type layer, a gallium nitride (GaN)-based material may be usedfor the active layer as well as the n-type layer, and there is not aparticular limitation to material combinations.

The each layer constituting the light emitting functional layer 14 iscomposed of a plurality of semiconductor single crystal grains, whereinthe layer has a single crystal structure in the direction approximatelynormal to the substrate. That is, each layer is composed of a pluralityof semiconductor single crystal grains connected two-dimensionally inthe direction of a horizontal plane, and, therefore, has a singlecrystal structure in the direction approximately normal to thesubstrate. Therefore, although each layer of the light emittingfunctional layer 14 is not a single crystal as a whole, it has a singlecrystal structure in terms of local domains, and can therefore havesufficiently high crystallinity for ensuring a light emitting function.Preferably, the semiconductor single crystal grains constituting therespective layers of the light emitting functional layer 14 have astructure in which grains are grown mostly in conformity with thecrystal orientation of the self-supporting polycrystalline galliumnitride substrate, which is the substrate 12. The “structure in whichgrains are grown mostly in conformity with the crystal orientation ofthe self-supporting polycrystalline gallium nitride substrate” means astructure resulting from crystal growth influenced by the crystalorientation of the self-supporting polycrystalline gallium nitridesubstrate, and it is not necessarily limited to a structure in whichgrains are grown completely in conformity with the crystal orientationof the self-supporting polycrystalline gallium nitride substrate, andmay be a structure in which grains are grown, to some extent, inconformity with the crystal orientation of the self-supportingpolycrystalline gallium nitride substrate as long as a desired lightemitting function can be ensured. That is, this structure also includesa structure in which grains are grown in crystal orientation differentfrom that of the oriented polycrystalline sintered body. In this sense,the expression “structure in which grains are grown mostly in conformitywith crystal orientation” can be paraphrased as “structure in whichgrains are grown in a manner mostly derived from crystal orientation”.Therefore, such crystal growth is preferably epitaxial growth, but it isnot limited thereto, and may take a variety of similar crystal growthforms. In particular, when the layers respectively constituting then-type layer, the active layer, the p-type layer, and the like grow inthe same crystal orientation as the self-supporting polycrystallinegallium nitride substrate, the structure is such that the crystalorientation from the self-supporting polycrystalline gallium nitridesubstrate to each layer of the light emitting functional layer is mostlyaligned with respect to the direction approximately normal to thesubstrate, and favorable light emitting properties can be obtained. Thatis, when the light emitting functional layer 14 also grows mostly inconformity with the crystal orientation of the self-supportingpolycrystalline gallium nitride substrate 12, the orientation is mostlyuniform in the direction perpendicular to the substrate. Accordingly, astate identical to a single crystal is attained in the direction normalto the substrate. Thus, a self-supporting polycrystalline galliumnitride doped with an n-type dopant makes it possible to form avertically-structured light emitting device including theself-supporting polycrystalline gallium nitride substrate as a cathode.On the other hand, a self-supporting polycrystalline gallium nitridesubstrate doped with a p-type dopant makes it possible to form avertically-structured light emitting device including theself-supporting polycrystalline gallium nitride substrate as an anode.

When at least the layers, such as the n-type layer, the active layer,and the p-type layer, constituting the light emitting functional layer14 grow in the same crystal orientation, each layer is observed as asingle crystal when viewed in the direction normal to the substrate, andthus it is also possible to recognize it as an aggregate ofsemiconductor single crystal grains having a columnar structure in whicha grain boundary is observed when the cross section in the direction ofa horizontal plane is viewed. Here, the “columnar structure” does notmean only a typical vertically long columnar shape, and is defined ashaving a meaning encompassing various shapes such as a horizontally longshape, a trapezoidal shape, and an inverted trapezoidal shape. Asdescribed above, each layer may have a structure in which grains aregrown, to some extent, in conformity with the crystal orientation of theself-supporting polycrystalline gallium nitride substrate, and does notnecessarily need to have a columnar structure in a strict sense. Asdescribed above, the growth of gallium nitride single crystal grains dueto the influence of the crystal orientation of the self-supportingpolycrystalline gallium nitride substrate, which is the substrate 12, isconsidered to be the cause of the columnar structure. Therefore, theaverage grain diameter at the cross section (hereinafter referred to asa cross-sectional average diameter) of semiconductor single crystalgrains that can also be called columnar structures is considered todepend on not only the conditions of film formation but also the averagegrain diameter at the plate surface of the self-supportingpolycrystalline gallium nitride substrate. The interface of columnarstructures constituting the light emitting functional layer influencesluminous efficiency and emission wavelength, and the presence of grainboundaries impairs light transmittance in the cross-sectional directionand causes light to be scattered or reflected. Therefore, in the case ofa structure from which light is extracted in the direction normal to thesubstrate, a luminance increasing effect due to scattered light fromgrain boundaries is also expected.

Crystallinity at the interface between columnar structures constitutingthe light emitting functional layer 14 is low, and therefore there is apossibility that the luminous efficiency deteriorates, the emissionwavelength changes, and the emission wavelength broadens. Therefore, alarger cross-sectional average diameter of the columnar structures ispreferable. Preferably, the cross-sectional average diameter of thesemiconductor single crystal grains at the outermost surface of thelight emitting functional layer 14 is 0.3 μm or greater, more preferably3 μm or greater, even more preferably 20 μm or greater, particularlypreferably 50 μm or greater, and most preferably 70 μm or greater.Although the upper limit of this cross-sectional average diameter is notparticularly defined, it is realistically 1000 μm or less, morerealistically 500 μm or less, and even more realistically 200 μm orless. In order to produce semiconductor single crystal grains havingsuch a cross-sectional average diameter, it is desirable that thecross-sectional average diameter at the outermost surface of thesubstrate of gallium nitride-based single crystal grains that constitutethe self-supporting polycrystalline gallium nitride substrate is 0.3 μmto 1000 μm and more desirably 3 μm or greater.

In the case where a material other than a gallium nitride (GaN)-basedmaterial is partially or entirely used for the light emitting functionallayer 14, a buffer layer may be provided between the self-supportingpolycrystalline gallium nitride substrate 12 and the light emittingfunctional layer 14 for inhibiting a reaction. The main component ofsuch a buffer layer is not particularly limited, and it is preferablethat the buffer layer is composed of a material, the main component ofwhich is at least one selected from zinc oxide (ZnO)-based materials andaluminum nitride (AlN)-based materials, and may suitably contain adopant for controlling it to be a p-type or an n-type.

It is preferable that each layer constituting the light emittingfunctional layer 14 is composed of a gallium nitride-based material. Forexample, an n-type gallium nitride layer and a p-type gallium nitridelayer may be grown in this order on the self-supporting polycrystallinegallium nitride substrate 12, or the order of stacking the p-typegallium nitride layer and the n-type gallium nitride layer may beinverse. Preferable examples of p-type dopants used for the p-typegallium nitride layer include one or more selected from the groupconsisting of beryllium (Be), magnesium (Mg), calcium (Ca), strontium(Sr), zinc (Zn), and cadmium (Cd). Preferable examples of n-type dopantsused for the n-type gallium nitride layer include one or more selectedfrom the group consisting of silicon (Si), germanium (Ge), tin (Sn), andoxygen (O). Moreover, the p-type gallium nitride layer and/or the n-typegallium nitride layer may be composed of gallium nitride formed into amixed crystal with a crystal of one or more selected from the groupconsisting of AlN and InN, and the p-type layer and/or the n-type layermay be this mixed-crystal gallium nitride doped with a p-type dopant oran n-type dopant. For example, doping Al_(x)Ga_(1-x)N, which is a mixedcrystal of gallium nitride and AlN, with Mg makes it possible to providea p-type layer, and doping Al_(x)Ga_(1-x)N with Si makes it possible toprovide an n-type layer. Forming gallium nitride into a mixed crystalwith AlN widens the band gap and makes it possible to shift the emissionwavelength toward the high energy side. Moreover, gallium nitride may beformed into a mixed crystal with InN, and this narrows the band gap andmakes it possible to shift the emission wavelength toward the low energyside. Between the p-type gallium nitride layer and the n-type galliumnitride layer, there may be an active layer composed of GaN or a mixedcrystal of GaN and one or more selected from the group consisting of AlNand InN, that has a smaller band gap than both layers. The active layerhas a structure that forms a double heterojunction with a p-type layerand an n-type layer, and a configuration in which this active layer ismade thin corresponds to the light emitting device having a quantum wellstructure, which is one form of a p-n junction, and luminous efficiencycan be further increased. Moreover, the active layer may be configuredto have a smaller band gap than either layer and be composed of GaN or amixed crystal of GaN and one or more selected from the group consistingof AlN and InN. Luminous efficiency can be further increased also bysuch a single heterojunction. The gallium nitride-based buffer layer maybe composed of non-doped GaN or n-type or p-type-doped GaN, may containAlN or InN having a close lattice constant, or may be a mixed crystalformed with GaN and one or more crystals selected from the groupconsisting of AlN and InN.

The light emitting functional layer 14 may be composed of a plurality ofmaterial systems selected from gallium nitride (GaN)-based materials,zinc oxide (ZnO)-based materials, and aluminum nitride (AlN)-basedmaterials. For example, a p-type gallium nitride layer and an n-typezinc oxide layer may be grown on the self-supporting polycrystallinegallium nitride substrate 12, and the order of stacking the p-typegallium nitride layer and the n-type zinc oxide layer may be inverse. Inthe case where the self-supporting polycrystalline gallium nitridesubstrate 12 is used as a part of the light emitting functional layer14, an n-type or p-type zinc oxide layer may be formed. Preferableexamples of p-type dopants used for the p-type zinc oxide layer includeone or more selected from the group consisting of nitrogen (N),phosphorus (P), arsenic (As), carbon (C), lithium (Li), sodium (Na),potassium (K), silver (Ag), and copper (Cu). Moreover, preferableexamples of n-type dopants used for the n-type zinc oxide layer includeone or more selected from the group consisting of aluminum (Al), gallium(Ga), indium (In), boron (B), fluorine (F), chlorine (CI), bromine (Br),iodine (I), and silicon (Si).

A method for forming films of the light emitting functional layer 14 andthe buffer layer is not particularly limited as long as it allows growthmostly in conformity with the crystal orientation of the self-supportingpolycrystalline gallium nitride substrate, and preferable examplesinclude vapor phase methods such as MOCVD, MBE, HVPE, and sputtering,liquid phase methods such as Na flux method, ammonothermal method,hydrothermal method, and sol-gel method, powder methods that utilizesolid phase growth of powder, and combinations of these. For example, inthe case where the light emitting functional layer 14 composed of agallium nitride-based material is produced using MOCVD, at least anorganic metal gas containing gallium (Ga) (such as trimethylgallium) anda gas containing at least nitrogen (N) (such as ammonia) as rawmaterials may be flown over a substrate to allow growth in, for example,an atmosphere containing hydrogen, nitrogen, or both within atemperature range of about 300 to 1200° C. In this case, film formationmay be performed by suitably introducing an organic metal gas containingindium (In) or aluminum (A1) for band gap control as well as silicon(Si) or magnesium (Mg) as an n-type and p-type dopant (such astrimethylindium, trimethylaluminum, monosilane, disilane, andbis-cyclopentadienylmagnesium).

Moreover, in the case where materials other than gallium nitride-basedmaterials are used for the light emitting functional layer 14 and thebuffer layer, a film of a seed crystal layer 3Q may be formed on theself-supporting polycrystalline gallium nitride substrate. A method forforming a film of the seed crystal layer and a material are notparticularly limited as long as crystal growth that is mostly inconformity with the crystal orientation is promoted. For example, when azinc oxide-based material is used for a part of or all of the lightemitting functional layer 14, an extremely thin zinc oxide seed crystalmay be produced using a vapor phase growth method such as MOCVD, MBE,HVPE, or sputtering.

An electrode layer 16 and/or a phosphor layer may be further provided onthe light emitting functional layer 14. As described above, since alight emitting device including the electroconductive self-supportingpolycrystalline gallium nitride substrate 12 can have a verticalstructure, an electrode layer 18 can also be formed on the bottomsurface of the self-supporting polycrystalline gallium nitride substrate12 as shown in FIG. 1, but the self-supporting polycrystalline galliumnitride substrate 12 itself may be used as an electrode, and in thiscase, it is preferable that an n-type dopant is added to theself-supporting polycrystalline gallium nitride substrate 12. Theelectrode layers 16 and 18 may be composed of known electrode materials,and configuring the electrode layer 16 on the light emitting functionallayer 14 to be a transparent conductive film such as ITO or a metalelectrode with a lattice structure or the like having a high apertureratio is preferable for being able to increase the efficiency ofextracting light produced in the light emitting functional layer 14.

When the light emitting functional layer 14 can emit ultraviolet light,a phosphor layer for converting ultraviolet light into visible light maybe provided on the outer side of the electrode layer. The phosphor layermay be a layer containing a known fluorescent component capable ofconverting ultraviolet rays into visible light, and is not particularlylimited. For example, preferable is such a configuration that afluorescent component that becomes excited by ultraviolet light andemits blue light, a fluorescent component that becomes excited byultraviolet light and emits blue to green light, and a fluorescentcomponent that becomes excited by ultraviolet light and emits red lightare allowed to be concomitantly present to obtain white light as a mixedcolor. Preferable combinations of such fluorescent components include(Ca,Sr)₅(PO₄)₃CI:Eu, BaMgAl₁₀O₁₇: Eu and Mn, and Y₂O₃S:Eu, and it ispreferable to disperse these components in a resin such as siliconeresin to form a phosphor layer. Such fluorescent components are notlimited to components exemplified above, and other ultraviolet-excitedphosphors such as yttrium aluminum garnet (YAG), silicate phosphors, andoxynitride-based phosphors may be combined.

On the other hand, when the light emitting functional layer 14 can emitblue light, a phosphor layer for converting blue light into yellow lightmay be provided on the outer side of the electrode layer. The phosphorlayer may be a layer containing a known fluorescent component capable ofconverting blue light into yellow light, and is not particularlylimited. For example, it may be a combination with a phosphor that emitsyellow light, such as YAG. Accordingly, a pseudo-white light source canbe obtained because blue light that has passed through the phosphorlayer and yellow light from the phosphor are complementary. The phosphorlayer may be configured to perform both conversion of ultraviolet lightinto visible light and conversion of blue light into yellow light byincluding both a fluorescent component that converts blue into yellowand a fluorescent component that converts ultraviolet light into visiblelight.

Applications

The self-supporting polycrystalline gallium nitride substrate of thepresent invention can be preferably used in not only the above-describedlight emitting device but also various applications such as variouselectronic devices, power devices, photodetectors, and solar cellwafers.

EXAMPLES

The present invention will now be more specifically described by way ofthe following examples.

Example A1 Ge-Doped Self-Supporting Polycrystalline Gallium NitrideSubstrate

(1) Production of c-Plane Oriented Alumina Sintered Body

As a raw material, a plate-shaped alumina powder (manufactured by KinseiMatec Co., Ltd., grade 00610) was provided. 7 parts by weight of abinder (polyvinyl butyral: lot number BM-2, manufactured by SekisuiChemical Co., Ltd.), 3.5 parts by weight of a plasticizer (DOP:di(2-ethylhexyl) phthalate, manufactured by Kurogane Kasei Co., Ltd.), 2parts by weight of a dispersing agent (Rheodol SP-O30, manufactured byKao Corporation), and a dispersion medium (2-ethylhexanol) were mixedwith 100 parts by weight of the plate-shaped alumina particles. Theamount of the dispersion medium was adjusted so that the slurryviscosity was 20000 cP. The slurry prepared as above was formed into asheet on a PET film by a doctor blade method so as to have a drythickness of 20 μm. The resulting tape was cut into circles having adiameter of 50.8 mm (2 inches), then 150 pieces were stacked and placedon an A1 plate having a thickness of 10 mm, and then vacuum packing wasperformed. This vacuum pack was subjected to isostatic pressing in hotwater at 85° C. under a pressure of 100 kgf/cm², and a disc-shaped greenbody was obtained.

The resulting green body was placed in a degreasing furnace anddegreased at 600° C. for 10 hours. The resulting degreased body wasfired in a hot press at 1600° C. for 4 hours under a surface pressure of200 kgf/cm² in nitrogen using a graphite mold. The resulting sinteredbody was placed on a graphite plate and re-fired at 1700° C. for 2 hoursunder a gas pressure of 1500 kgf/cm² in argon in a hot isostatic press(HIP).

The sintered body obtained in this way was fixed to a ceramic surfaceplate and ground to #2000 using grinding wheel to flatten the platesurface. Next, the plate surface was smoothed by lapping using diamondabrasive grains to obtain an oriented alumina sintered body having adiameter of 50.8 mm (2 inches) and a thickness of 1 mm as an orientedalumina substrate. Flatness was improved by reducing the size ofabrasive grains from 3 μm to 0.5 μm in a stepwise manner. The averageroughness Ra after processing was 4 nm.

(2) Production of Ge-Doped Self-Supporting Polycrystalline GalliumNitride Substrate (2 a) Film Formation of Seed Crystal Layer

Next, a seed crystal layer was formed on the processed oriented aluminasubstrate using MOCVD. Specifically, a 40 nm thick low-temperature GaNlayer was deposited at 530° C., and then a GaN film having a thicknessof 3 μm was laminated at 1050° C. to obtain a seed crystal substrate.

(2 b) Film Formation of Ge-Doped GaN Layer by Na Flux Method

The seed crystal substrate produced in the above process was placed inthe bottom of a cylindrical, flat-bottomed alumina crucible having aninner diameter of 80 mm and a height of 45 mm, and then the crucible wasfilled with a melt composition in a glovebox. The composition of themelt composition is as follows.

-   -   Metal Ga: 60 g    -   Metal Na: 60 g    -   Germanium tetrachloride: 1.85 g

This alumina crucible was put in a vessel made of a refractory metal andsealed, and then placed on a rotatable rack of a crystal growth furnace.After the temperature and the pressure were increased to 870° C. and 4.0MPa in a nitrogen atmosphere, the melt was maintained for 30 hours whilebeing rotated and stirred, and gallium nitride crystals were allowed togrow. After the end of crystal growth, the growth vessel was cooledslowly back to room temperature for 3 hours, and then the growth vesselwas taken out from the crystal growth furnace. The melt compositionremaining in the crucible was removed using ethanol, and a sample inwhich gallium nitride crystals grew was recovered. In the resultingsample, Ge-doped gallium nitride crystals grew on the entire surface ofthe 50.8 mm (2 inches) seed crystal substrate, and the crystal thicknesswas about 0.3 mm. No cracks were observed.

The oriented alumina substrate portion of the sample obtained in thisway was removed by grinding with grinding wheel to obtain a Ge-dopedgallium nitride single body. The plate surface of the Ge-doped galliumnitride crystals was polished to flatten the plate surface. Furthermore,the plate surface was smoothed by lapping and CMP to obtain a Ge-dopedself-supporting polycrystalline gallium nitride substrate having athickness of about 130 μm. The self-supporting polycrystalline galliumnitride substrate after processing had an average roughness Ra of 0.2nm.

Although an n-type semiconductor was produced by germanium doping inthis example, doping may be performed using another element or dopingmay not be performed depending on the application and the structure.

(Evaluation of Cross-Sectional Average Diameter of Self-SupportingPolycrystalline Gallium Nitride Substrate)

In order to measure the cross-sectional average diameter of GaN singlecrystal grains at the outermost surface of the self-supportingpolycrystalline gallium nitride substrate, an image of the top surfaceof the self-supporting substrate was taken with a scanning electronmicroscope. The visual field range was determined in such a way thatwhen straight lines were diagonally drawn on the obtained image, thestraight lines crossed 10 to 30 columnar structures. The cross-sectionalaverage grain diameter of GaN single crystal grains at the outermostsurface of the self-supporting polycrystalline gallium nitride substratewas determined by diagonally drawing two straight lines on the obtainedimage, taking the average of the line segment lengths inside all grainscrossed by the straight lines, and multiplying the average by 1.5.

Moreover, as a result of measuring the cross-sectional average diameterof GaN single crystal grains at the top surface and the bottom surfaceof the self-supporting polycrystalline gallium nitride substrate using amethod as described above, the cross-sectional average diameter at thetop surface was about 76 μm, and the cross-sectional average diameter atthe bottom surface was about 51 μm. In this way, the cross-sectionalaverage diameter was larger at the top surface than the bottom surface,and D_(T)/D_(B), which is the ratio of the cross-sectional averagediameter D_(T) at the substrate top surface to the cross-sectionalaverage diameter D_(B) of the substrate bottom surface, was about 1.5.Moreover, the aspect ratio of GaN single crystal grains calculated asthe ratio of the thickness T of GaN crystals to the cross-sectionalaverage diameter D_(T) at the top surface was about 1.7. In thisexample, it was possible to clearly determine the interface on thescanning microscope image of the top surface, but the above evaluationmay be carried out after performing processing to emphasize theinterface by thermal etching or chemical etching. Also, theabove-described evaluation may be performed using a crystal grain mapfrom EBSD measurement, which will be described below.

(Cross-Sectional EBSD Measurement of Gallium Nitride Crystals)

Inverse pole figure mapping of the plate surface of the self-supportingpolycrystalline gallium nitride substrate was performed using an SEM(manufactured by JEOL Ltd., JSM-7000F) equipped with an electronbackscatter diffraction (EBSD) system (manufactured by TSL Solutions,OIM) in a visual field of 300 μm×300 μm. This EBSD measurement wasperformed under the following conditions:

<EBSD Measurement Conditions>

-   -   Accelerating voltage: 15 kV    -   Irradiation beam current: 2×10⁻⁸A    -   Working distance: 15 mm    -   Step size: 2 μm    -   Measurement program: OIM Data Collection

FIG. 2 shows the resulting inverse pole figure map. FIG. 3 shows thefrequency distribution of tilt angles from the c-axis direction ofgrains constituting the outermost surface calculated from the inversepole figure map. The inverse pole figure map was subjected to imagecleanup according to the Grain Dilation method using the analysissoftware OIM Analysis. The tilt angle frequency was calculated aftercleanup. The cleanup conditions were as follows.

<Cleanup Conditions During EBSD Analysis>

-   -   Grain tolerance angle: 5°    -   Minimum grain size: 2 pixels

The grains constituting gallium nitride crystals were oriented in amanner such that their c-planes faced to the direction normal to thesubstrate. The average tilt angle of the grains constituting theoutermost surface was 5.0°, and the distribution appeared similar to theGaussian distribution, with the frequency of grains tilted at an angleof 1 to 10° being 85%_(.)

(Defect Density Evaluation by CL Measurement)

The defect density of the self-supporting polycrystalline galliumnitride substrate was determined by counting the number of dark spots,which appear darker than the surroundings due to their weak lightemission in a cathode luminescence (CL) image, as dislocations appearingon the plate surface of the self-supporting polycrystalline galliumnitride substrate. In the present invention, defect density measurementby the CL image was performed at an accelerating voltage of 15 kV usingan SEM (manufactured by Hitachi High-Technologies Corporation, S-3400NType E) equipped with a cathode luminescence detector (manufactured byGatan, Mini CL).

Two-hundred 80 μm×100 μm visual fields were examined by the CL method,but no clear dark spots were recognized in the gallium nitride crystals.That is, the defect density was approximately 0 defects/cm².

Example A2 Production of Ge-Doped Self-Supporting PolycrystallineGallium Nitride Substrate

(1) Production of c-Plane Oriented Alumina Sintered Body

A plate-shaped alumina powder (manufactured by Kinsei Matec Co., Ltd.,grade 02025) and a fine alumina powder (manufactured by Taimei ChemicalsCo., Ltd., grade TM-DAR) were provided as raw materials, and 50 parts byweight of the plate-shaped alumina powder and 50 parts by weight of thefine alumina powder were mixed to obtain an alumina raw material. Next,8 parts by weight of a binder (polyvinyl butyral: product name BM-2,manufactured by Sekisui Chemical Co., Ltd.), 4 parts by weight of aplasticizer (DOP: di(2-ethylhexyl) phthalate, manufactured by KuroganeKasei Co., Ltd.), 2 parts by weight of a dispersing agent (RheodolSP-O30, manufactured by Kao Corporation), and a dispersion medium (amixture of xylene and 1-butanol in a weight ratio of 1:1) were mixedwith 100 parts by weight of the alumina raw material. The amount of thedispersion medium was adjusted so that the slurry viscosity was 20000cP. The slurry prepared as above was formed into a sheet on a PET filmby a doctor blade method so as to have a thickness after drying of 100μm. The resulting tape was cut into circles having a diameter of 50.8 mm(2 inches), then 30 pieces were stacked and placed on an A1 plate havinga thickness of 10 mm, and then vacuum packing was performed. This vacuumpack was subjected to isostatic pressing in hot water at 85° C. under apressure of 100 kgf/cm², and a disc-shaped green body was obtained. Theresulting green body was placed in a degreasing furnace and degreased at600° C. for 10 hours. The resulting degreased body was fired with a hotpress at 1700° C. for 4 hours under a surface pressure of 200 kgf/cm² innitrogen using a graphite mold.

The sintered body obtained in this way was fixed to a ceramic surfaceplate and ground to #2000 using grinding wheel to flatten the platesurface. Next, the plate surface was smoothed by lapping using diamondabrasive grains to obtain an oriented alumina sintered body having adiameter of 50.8 mm (2 inches) and a thickness of 1 mm as an orientedalumina substrate. Flatness was improved by reducing the size ofabrasive grains from 3 μm to 0.5 μm in a stepwise manner. The averageroughness Ra after processing was 4 nm.

(2) Production of Ge-Doped Self-Supporting Polycrystalline GalliumNitride Substrate

A Ge-doped GaN film having a thickness of about 0.3 mm was formed on anoriented alumina substrate by a method similar to Example A1. In theresulting sample, Ge-doped gallium nitride crystals grew on the entiresurface of the 50.8 mm (2 inches) seed crystal substrate, and thecrystal thickness was about 0.3 mm. No cracks were observed.

The oriented alumina substrate portion of the sample obtained in thisway was removed by grinding with grinding wheel to obtain a Ge-dopedgallium nitride single body. The plate surface of the Ge-doped galliumnitride crystals was polished to flatten the plate surface. Furthermore,the plate surface was smoothed by lapping and CMP to obtain a Ge-dopedself-supporting polycrystalline gallium nitride substrate having athickness of about 60 μm. The average roughness Ra of the surface of theself-supporting polycrystalline gallium nitride substrate afterprocessing was 0.5 nm.

As a result of measuring the cross-sectional average diameter of GaNsingle crystal grains at the top surface and the bottom surface of theself-supporting polycrystalline gallium nitride substrate using the samemethod as Example A1, the cross-sectional average diameter at the topsurface was about 20 μm, and the cross-sectional average diameter at thebottom surface was about 9 μm. In this way, the cross-sectional averagediameter was larger at the top surface than the bottom surface, andD_(T)/D_(B), which is the ratio of the cross-sectional average diameterD_(T) at the substrate top surface to the cross-sectional averagediameter D_(B) of the substrate bottom surface, was about 2.2. Moreover,the aspect ratio of GaN single crystal grains calculated as the ratio ofthe thickness T of GaN crystals to the cross-sectional average diameterD_(T) at the top surface was about 3.

As a result of performing EBSD measurement on the plate surface usingthe same method as Example A1, the grains constituting gallium nitridecrystals were oriented in a manner such that their c-planes faced to thedirection normal to the substrate, the average tilt angle of the grainsconstituting the outermost surface was 8.4°, and the frequency of grainstilted at an angle of 1 to 10° was 80%. The defect density determined bythe same method as Example A1 was 6×10¹ defects/cm².

Example A3 Comparative Production of Ge-Doped Self-SupportingPolycrystalline Gallium Nitride Substrate

(1) Production of c-Plane Oriented Alumina Sintered Body

99.8 parts by weight of a fine alumina powder (manufactured by TaimeiChemicals Co., Ltd., Grade TM-DAR) and 0.2 parts by weight of a yttriapowder (manufactured by Shin-Etsu Chemical Co. Ltd., Grade UU) weremixed, water was added as a solvent in a proportion of 50 cc per 100 gof the mixed powder, and the mixture was ground for 40 hours in a ballmill to form a slurry. The resulting slurry was poured into a plastermold having an inner diameter of 50 mm and placed in a magnetic field of12 T for 3 hours for slipcasting. The green body was removed from theplaster mold, dried at room temperature, and then fired in a hot pressat 1400° C. for 4 hours under a surface pressure of 200 kgf/cm² innitrogen using a graphite mold.

The sintered body obtained in this way was fixed to a ceramic surfaceplate and ground to #2000 using a grinding wheel to flatten the platesurface. Next, the plate surface was smoothed by lapping using diamondabrasive grains to obtain an oriented alumina sintered body having adiameter of 50.8 mm (2 inches) and a thickness of 1 mm as an orientedalumina substrate. Flatness was improved by reducing the size ofabrasive grains from 3 μm to 0.5 μm in a stepwise manner. The averageroughness Ra after processing was 4 nm.

(2) Production of Ge-Doped Self-Supporting Polycrystalline GalliumNitride Substrate

A Ge-doped GaN film having a thickness of about 0.3 mm was formed on theoriented alumina substrate by a method similar to Example A1. In theresulting sample, Ge-doped gallium nitride crystals grew on the entiresurface of the 50.8 mm (2 inches) seed crystal substrate, and thecrystal thickness was about 0.3 mm. No cracks were observed.

The oriented alumina substrate portion of the sample obtained in thisway was removed by grinding with grinding wheel to obtain a Ge-dopedgallium nitride single body. The plate surface of the Ge-doped galliumnitride crystals was polished to flatten the plate surface. Furthermore,the plate surface was smoothed by lapping and CMP to obtain a Ge-dopedself-supporting polycrystalline gallium nitride substrate having athickness of about 70 μm. The average roughness Ra of the surface of theself-supporting polycrystalline gallium nitride substrate afterprocessing was 0.5 nm.

As a result of measuring the cross-sectional average diameter of GaNsingle crystal grains at the top surface and the bottom surface of theself-supporting polycrystalline gallium nitride substrate using the samemethod as Example A1, the cross-sectional average diameter at the topsurface was about 9 μm, and the cross-sectional average diameter at thebottom surface was about 8 μm. The ratio D_(T)/D_(B) was about 1.1,which is the ratio of the cross-sectional average diameter D_(T) of thesubstrate top surface to the cross-sectional average diameter D_(B) ofthe substrate bottom surface. Moreover, the aspect ratio of GaN singlecrystal grains calculated as the ratio of the thickness T of GaNcrystals to the cross-sectional average diameter D_(T) at the topsurface was about 7.8.

As a result of performing EBSD measurement on the plate surface usingthe same method as Example A1, the grains constituting gallium nitridecrystals were oriented in a manner such that their c-planes faced to thedirection normal to the substrate, the average tilt angle of the grainsconstituting the outermost surface was 0.8°, and the frequency of grainstilted at an angle of 1 to 10° was 74%. The defect density determined bythe same method as Example A1 was 2×10⁶ defects/cm².

Example B1 Light Emitting Device Including Ge-Doped Self-SupportingPolycrystalline Gallium Nitride Substrate

(1) Production of Light Emitting Device

Using MOCVD, a 1 μm thick n-GaN layer doped to give a Si atomconcentration of 5×10¹⁶/cm³ was deposited at 1050° C. as an n-type layeron each Ge-doped self-supporting polycrystalline gallium nitridesubstrate produced in Examples A1 to A3. Next, a multiple quantum welllayer was deposited at 750° C. as a light emitting layer. Specifically,five 2.5 nm thick InGaN well layers and six 10 nm thick GaN barrierlayers were alternately stacked. Next, a 200 nm thick p-GaN doped togive a Mg atom concentration of 1×10¹⁹/cm³ was deposited at 950° C. as ap-type layer. Thereafter, the sample was taken out from the MOCVDapparatus, and 800° C. heat treatment was performed for 10 minutes in anitrogen atmosphere as activation treatment of Mg ions of the p-typelayer.

Next, using a photolithography process and a vacuum deposition method,Ti/Al/Ni/Au films as a cathode were patterned on the surface on the sideopposite to the n-GaN layer and the p-GaN layer of the self-supportingpolycrystalline gallium nitride substrate in a thickness of 15 nm, 70nm, 12 nm, and 60 nm, respectively. Thereafter, to improve ohmic contactcharacteristics, 700° C. heat treatment was performed in a nitrogenatmosphere for 30 seconds. Furthermore, using a photolithography processand a vacuum deposition method, Ni/Au films were patterned as atranslucent anode on the p-type layer in a thickness of 6 nm and 12 nm,respectively. Thereafter, to improve ohmic contact characteristics, 500°C. heat treatment was performed in a nitrogen atmosphere for 30 seconds.Furthermore, using a photolithography process and a vacuum depositionmethod, Ni/Au films that served as an anode pad were patterned in athickness of 5 nm and 60 nm, respectively, on a partial area of the topsurface of the aforementioned Ni/Au films as a translucent anode. Thewafer obtained in this way was cut into a chip and, further, furnishedwith a lead frame to obtain a light emitting device having a verticalstructure.

(2) Evaluation of Light Emitting Device

When electricity was applied across the cathode and the anode and I-Vmeasurement was performed, devices with any of the substrates ofExamples A1 to A3 demonstrated rectifying characteristics. Moreover,with an electric current flowing in the forward direction, emission oflight having a wavelength of 450 nm was confirmed. The device with thesubstrate of Example A1 had the highest luminance, the luminance of thedevice with the substrate of Example A2 was lower than that of ExampleA1 but still acceptable, and the device with the substrate of Example A3had a markedly lower luminance than that of Example A2.

What is claimed is:
 1. A self-supporting polycrystalline gallium nitridesubstrate composed of a plurality of gallium nitride-based singlecrystal grains having a specific crystal orientation in a directionapproximately normal to the substrate, wherein crystal orientations ofindividual gallium nitride-based single crystal grains as determinedfrom inverse pole figure mapping by electron backscatter diffraction(EBSD) analysis performed on a substrate surface are distributed withvarious tilt angles from the specific crystal orientation, wherein anaverage tilt angle thereof is 1 to 10°.
 2. The self-supportingpolycrystalline gallium nitride substrate according to claim 1, whereinno less than 80% of the gallium nitride-based single crystal grainssubjected to inverse pole figure mapping by electron backscatterdiffraction (EBSD) analysis have a tilt angle within a range of 1 to10°.
 3. The self-supporting polycrystalline gallium nitride substrateaccording to claim 1, wherein the tilt angle of the galliumnitride-based single crystal grains is distributed according to Gaussiandistribution.
 4. The self-supporting polycrystalline gallium nitridesubstrate according to claim 1, having a defect density of 1×10⁴defects/cm² or less.
 5. The self-supporting polycrystalline galliumnitride substrate according to claim 1, having a defect density of 1×10²defects/cm² or less.
 6. The self-supporting polycrystalline galliumnitride substrate according to claim 1, having a single crystalstructure in the direction approximately normal to the substrate.
 7. Theself-supporting polycrystalline gallium nitride substrate according toclaim 1, wherein the gallium nitride-based single crystal grains exposedat a top surface of the self-supporting polycrystalline gallium nitridesubstrate connect to a bottom surface of the self-supportingpolycrystalline gallium nitride substrate without intervention of agrain boundary.
 8. The self-supporting polycrystalline gallium nitridesubstrate according to claim 1, wherein a cross-sectional averagediameter D_(T) at an outermost surface of the gallium nitride-basedsingle crystal grains exposed at a top surface of the self-supportingpolycrystalline gallium nitride substrate is different from across-sectional average diameter D_(B) at an outermost surface of thegallium nitride-based single crystal grains exposed at a bottom surfaceof the self-supporting polycrystalline gallium nitride substrate.
 9. Theself-supporting polycrystalline gallium nitride substrate according toclaim 1, wherein a ratio D_(T)/D_(B) is greater than 1.0, which is aratio of a cross-sectional average diameter D_(T) at an outermostsurface of the gallium nitride-based single crystal grains exposed at atop surface of the self-supporting polycrystalline gallium nitridesubstrate to a cross-sectional average diameter D_(B) at an outermostsurface of the gallium nitride-based single crystal grains exposed at abottom surface of the self-supporting polycrystalline gallium nitridesubstrate.
 10. The self-supporting polycrystalline gallium nitridesubstrate according to claim 1, wherein the gallium nitride-based singlecrystal grains have a cross-sectional average diameter of 10 μm orgreater at an outermost surface of the substrate.
 11. Theself-supporting polycrystalline gallium nitride substrate according toclaim 1, having a thickness of 20 μm or greater.
 12. The self-supportingpolycrystalline gallium nitride substrate according to claim 1, having adiameter of 50.8 mm or greater.
 13. The self-supporting polycrystallinegallium nitride substrate according to claim 1, wherein the galliumnitride-based single crystal grains are doped with an n-type dopant or ap-type dopant.
 14. The self-supporting polycrystalline gallium nitridesubstrate according to claim 1, wherein the gallium nitride-based singlecrystal grains are free from a dopant.
 15. The self-supportingpolycrystalline gallium nitride substrate according to claim 1, whereinthe gallium nitride-based single crystal grains are made of a mixedcrystal.
 16. The self-supporting polycrystalline gallium nitridesubstrate according to claim 1, wherein the gallium nitride-based singlecrystal grains constituting the self-supporting polycrystalline galliumnitride substrate are crystallographically non-oriented in a directionparallel to the plate surface, which is perpendicular to the directionnormal to the substrate.
 17. A light emitting device comprising: theself-supporting polycrystalline gallium nitride substrate according toclaim 1; and a light emitting functional layer formed on the substrate,wherein the light emitting functional layer has at least one layercomposed of a plurality of semiconductor single crystal grains, whereinthe at least one layer has a single crystal structure in a directionapproximately normal to the substrate.